Amir Hossein Saeidnejad - Assistant Professor, Department of Agriculture, Payame Noor University (PNU), Iran

Water is involved in all components of the climate system (atmosphere, hydrosphere, cryosphere, land surface and biosphere). Therefore, climate change affects water through a number of mechanisms. Climate warming observed over the past several decades is consistently associated with changes in a number of components of the hydrological cycle and hydrological systems such as: changing precipitation patterns, intensity and extremes; widespread melting of snow and ice; increasing atmospheric water vapour; increasing evaporation; and changes in soil moisture and runoff. There is significant natural variability on interannual to decadal time-scales – in all components of the hydrological cycle, often masking long-term trends. There is still substantial uncertainty in trends of hydrological variables because of large regional differences, and because of limitations in the spatial and temporal coverage of monitoring networks. With the focus on precipitations, it was observed that Precipitation over land generally increased over the 20th century between 30°N and 91°N, but notable decreases have occurred in the past 30–40 years from 00°S to 30°N. Generally, the attribution of changes in global precipitation is uncertain, since precipitation is strongly influenced by largescale patterns of natural variability. Considering the evapotranspiration, there are very limited direct measurements of actual evapotranspiration over global land areas, while global analysis products are sensitive to the type of analysis and can contain large errors, and thus are not suitable for trend analysis. Therefore, there is little literature on observed trends in evapotranspiration, whether actual or potential. Further investigation regards to the specified time and location for water resources availability is recommended.

Evapotranspiration, Hydrological systems, Precipitation